Select users based on their date joined and date left - sql

Question

I have the following table:
Person
UserID Name Date_Joined Date_Left
1 Test 2018-08-10 NULL
2 Test2 2018-07-10 NULL
3 Test3 2018-07-10 2018-12-31
4 Test4 2018-08-10 2018-09-10
I want to check by only their join and/or left date if they are billable(=active) or not.
These are billable users:
User whose start month is at least one month before billdate
with no date left
User whose start month is at least one month before billdate
with date left month equal to the billdate or later than the
billdate
Month of billing = always the previous month.
I use the following query:
DECLARE #billdate AS DATE = '2018-09-01';
SELECT *
FROM person
WHERE CompanyID = 1205 AND (
(
date_joined <= EOMONTH(#billdate, -1)
)
OR
(
date_left > EOMONTH(#billdate, -1) AND
date_left <= EOMONTH(#billdate)
)
)
My problems:
User Test4 is still present in my table if I set the billdate to 2018-11-01.
User Test3 dissapears if i set billdate to 2019-01-01
What is wrong with my query and how can I optimize this?
Sample data:
User list:
1 - Test - 2018-08-10 - NULL
2 - Test2 - 2018-07-10 - NULL
3 - Test3 - 2018-07-10 - 2018-12-31
4 - Test4 - 2018-08-10 - 2018-09-10
For the bill period of the previous month (= 8 / August) = #billdate 2018-09-10, these are the billable users:
Test2
Test3
However, when I change the bill period to 10 / october, these are the billable users:
Test
Test2
Test3

Answer 1

If I understand your logic correctly, a billable user is one whose start month is at least one month before the current month, and whose end month is less than or equal to the current month.
SELECT *,
CASE WHEN #billdate >= DATEADD(d, 1, EOMONTH(date_joined)) AND
(#billdate <= DATEADD(d, 1, EOMONTH(date_left)) OR date_left IS NULL)
THEN 'billable'
ELSE 'not billable' END AS status
FROM person;
This billing logic appears to be consistent with customers getting the first month free, but being billable from the second up to the final month.
Demo

Answer 2

use make flag comparing join month and year with bill month and year and create a flag for each user then use sub-query for filter
select * from t
(
select *,case when (month(date_joined)=month(billdate)
and year(date_joined)=year(billdate)) then 'N' else 'Y' end as flag_billable
from user
) as t where t.flag_billable='Y'
For your query
DECLARE #billdate AS DATE = '2018-09-01';
select * from
(
SELECT * ,case when (month(date_joined)=month(#billdate)
and year(date_joined)=year(#billdate)) then 'N' else 'Y' end as flag_billable
FROM person
) t where t.flag_billable='Y'